Explosive eruption at Calbuco Volcano in southern Chile

The Southern Chilean volcanic arc produced a highly-explosive eruption. An eruption column rose from the 2,000 m high summit crater of Calbuco Volcano 15 km into the atmosphere.

Article updated on April 27, 2015

So far, Calbuco released an amount of 0.21 km3 of tephra. Volumewise, this corresponds to a VEI of 4. Now the slopes of the volcanic edifice are covered with loose pyroclastic rocks. If rain sets in, this unconsolidated material may become remobilised and may flow downwards through the valleys. Sekundary lahars may form – those are variably concentrated volcanic mass flows consisting of rock fragments, sediments, and water. For the next days, the Chilean Meteorological Service forecasts variable weather conditions, including overcast periods, but also clouds and rain showers. Under consideration of the expected precipitation, the Chilean Geological Survey Sernageomín has evaluated the risk of upcoming lahars. The results of the analysis are presented in a new risk map. Therein, the area to the northeast of the volcano are most exposed to lahar hazards.

The eruption itself has claimed no lifes yet. By observing how the eruption evolves and thus by permanently updating the risk assessment, the local authorities are prepared for potentially upcoming secondary phenomena.

Shortly before the eruption onset, the Chilean Volcano Observatory of the Southern Andes (OVDAS) registered signs of an impending eruption. The network of seismic stations installed at Calbuco Volcano displayed changes in seismic activity. The type of seismic swarm indicated that moving magma was penetrating the Earth's crust. The first stronger earthquake occurred beneath the western slope of the volcano at a depth of 7.4 km.

Then, an impressive eruption column of 15 km height built up above Calbuco's main crater, accompanied by hundreds of earthquakes. The ash cloud dispersed mainly to the North and to the East. Collapes of the eruption column formed pyroclastic flows that swept down the slopes of the volcanic edifice. Sernageomin, the Chilean Geological Survey, ordered evacuation and closed an area of 10 km around the crater. After the second, more violent pulse of the eruption, the restricted area was enlarged to a radius of 20 km. Although the explosiveness waned after the second activity peak, the alert level of red has been maintained, and the air space affected by the ash cloud has been closed for air traffic.

Calbuco is a very explosive subduction zone volcano, situated in the Chilean Southern Volcanic Zone on the South American Plate, above the subducting Nazca Plate. Several episodes of eruptive activity since the Late-Pleistocene have constructed a stratovolcano that is truncated at the top. Its series of historical eruptions only spans over the past 120 years. In contrast to Villarrica Volcano 220 km farther north, which has produced smaller eruptions at a high frequency, Calbuco has generated rather medium-size eruptions at a lower frequency during historical times. It has been quiet for the past 43 years. Derived from the temporal distribution of the eruptions in Calbuco's historical record, however, the present-day eruption is not unexpected. The volcano's surroundings are densely populated and frequented by visitors. In addition to a number of townships in the immediate vicinity, the cities Puerto Montt and Puerto Varas are only 30 km away from Calbuco.

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